The present invention relates to protective barriers for photomasks, and more particularly to a barrier for minimizing the contamination of a photomask during the removal and replacement of a pellicle.
In the semi-conductor chip industry it is well known that pattern transfer from a photomask to a substrate is accomplished by exposing a mask to a light source. During the pattern transfer process, also called the photolithographic process, patterns on the photomask are projected onto the substrate which has been treated with a photo-sensitive sensitive substance. This results in the mask etchings being reproduced onto the substrate. Unfortunately, any foreign substance on the surface of the mask will also be reproduced on the substrate and therefore will interfere with proper pattern transfer to the substrate.
To eliminate contamination of the mask surface, a framed, thin membrane known as a pellicle is mounted on the mask surface, such that the pellicle membrane extends parallel to the mask at a predetermined distance spaced away from it. Any contamination which would ordinarily land of the mask surface instead falls on the pellicle membrane.
Pellicles substantially eliminate the above problem because contamination on the pellicle membrane will not be projected onto the substrate. The frame of the pellicle supports the membrane at a distance spaced away from the mask surface so any particles or other contaminants on the pellicle membrane will be out of focus during pattern transfer.
Pellicle frames, with the pellicle membrane already attached, are mounted directly to the surface of the mask by use of a double-sided adhesive. When it becomes necessary to replace the pellicle, the entire pellicle is removed from the mask. The removal of the pellicle often generates particulate contaminants. Some possible contaminants include small fragments of dried adhesive loosened and exposed by the removal of the pellicle, and particulate generated by mechanical contact between tools used to remove the pellicle and the pellicle frame. These contaminants are generated close to the surface of the mask, and can reach the pattern of the photomask by simple lateral movement. Thus, following the changing of a pellicle, cleaning and reinspection of the mask may be required to ensure that its original quality has not been affected by the pellicle changing process. This increases the cost of pellicle replacement in terms of time, money, and potential damage to the mask.
Solutions to the contamination problem in the prior art include cleaning the mask after changing the pellicle, and utilizing a removable pellicle mounting system. Cleaning has the disadvantage of potentially increasing contamination. Thus, masks that have been cleaned require reinspection. Another solution is offered by the use of a removable pellicle as disclosed in U.S. Pat. No. 4,737,387 to Yen. The invention in the '387 patent comprises a mounting frame attached to a photomask surface by an adhesive, onto which a pellicle is mounted by an adhesive. The pellicle can be removed from the mounting frame without disrupting the adhesive bond between the mounting frame and the photomask. Additionally, the mounting frame extends somewhat above the surface of the photomask, providing a physical impediment that particulate must surmount to reach the photomask pattern. However, this solution still poses problems. First, because the mounting frame mounts the pellicle on a shoulder of the frame, both the lower face and the inside face of the pellicle frame are in mechanical and perhaps adhesive contact with the mounting frame. The amount of contact area between a pellicle mounted to a mounting frame and the mounting frame itself is greater than the contact area between a pellicle adhered directly to a photomask and the photomask. Thus, when the pellicle is removed from the mounting frame, a greater surface area of the pellicle frame is mechanically disrupted, and perhaps a greater area of adhesive residue is exposed, by the removal process than would be if the pellicle had been directly mounted to and removed from the photomask. The result is a potential increase in the amount of particulate generated by the changing of a pellicle.
Another disadvantage to the use of a removable pellicle as disclosed in the '387 patent to Yen is that the pellicle mounting frame works with only one size or shape of pellicle. If a different pellicle is desired, the mounting frame must be removed and replaced. The removal and replacement of a mounting frame might create an additional source of contamination and increase the cost of changing pellicle sizes or shapes.
Yet another disadvantage to the removable pellicle of the '387 patent to Yen is that the pellicle frame is in contact with the mounting frame for the entire height of the mounting frame shoulder. Thus, upon the removal of a pellicle, particulate may be generated at the highest point of the mounting frame shoulder. Particulate generated at this point encounter no physical barrier between the point of generation and the photomask pattern, and therefore may easily contaminate the pattern.
Accordingly, it is a general object of the present invention to provide an improved mechanism and method for preventing the contamination of a photomask pattern by particulate generated during the removal and replacement of a pellicle. More specific objects of the present invention are to provide a mechanism and method which are simple to fabricate and more readily adaptable to the use of pellicles of different sizes and shapes.